Isolate clients and documents for each project

[hackerwins]

What this PR does / why we need it:

Isolate clients and documents for each project

• Add ProjectID to in DocInfo and ClientInfo.
• It is not compatible with the existing MongoDB and the migration script was not written as it was pre-deployment.
• Keys for clients and documents are unique for each project rather than a system global.
• The logic to check whether the Document and Client belong to the Project is in the database layer.

This PR also cleans up codes

• Rename db package to database
• Extract Paging
• Move project logic to projects package
• Remove unused codes

Which issue(s) this PR fixes:

Fixes #310

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #319 (2fc43f6) into main (149693a) will decrease coverage by 0.52%.
The diff coverage is 44.11%.

@@            Coverage Diff             @@
##             main     #319      +/-   ##
==========================================
- Coverage   46.76%   46.24%   -0.53%     
==========================================
  Files          58       62       +4     
  Lines        5061     5173     +112     
==========================================
+ Hits         2367     2392      +25     
- Misses       2439     2521      +82     
- Partials      255      260       +5     

Impacted Files	Coverage Δ
server/backend/backend.go	0.00% <0.00%> (ø)
server/backend/database/change_info.go	29.16% <ø> (ø)
server/backend/database/client_info.go	35.59% <0.00%> (ø)
server/backend/database/doc_info.go	0.00% <0.00%> (ø)
server/backend/database/mongo/client.go	0.00% <0.00%> (ø)
server/backend/database/mongo/config.go	61.90% <ø> (ø)
server/backend/database/mongo/encoder.go	0.00% <ø> (ø)
server/backend/database/mongo/indexes.go	0.00% <ø> (ø)
server/backend/database/mongo/registry.go	100.00% <ø> (ø)
server/backend/database/project_info.go	8.82% <ø> (ø)
... and 10 more

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